Optics and Lasers in Engineering 50 (2012) 1–7

Contents lists available at SciVerse ScienceDirect

Optics and Lasers in Engineering

0143-81

doi:10.1

journal homepage: www.elsevier.com/locate/optlaseng

Editorial

Flow visualization and beyond

1. Introduction

There has always been a strong connection between optics andflow visualization: the two fields obviously share a ‘‘visualapproach’’ and many flow visualization techniques are ‘‘optical’’by nature. Furthermore, as Milton Van Dyke wrote in his seminal1982 book [1]: ‘‘We who work in fluid mechanics are fortunate, as

are our colleagues in a few other fields such as optics, that our

subjects are easily visualized’’.According to Peter Freymuth [2] ‘‘Flow visualization is the art

and science of obtaining a clear image of a physical flow field and the

ability to capture it on sketch, photograph, or other video storage

device for display or further processing’’.Interest in flow visualization has constantly grown in the last

three decades: experimental techniques have developed tremen-dously, therefore we believe that the topic deserves a particularattention. In the following, before briefly presenting the papers ofthis special issue, we try to sketch a historical perspective as wellas to describe present features and possible future developmentsof flow visualization.

2. From Leonardo to modern wine optimization

During the Eighties of the last century, the supporters of the ‘‘allnumerical calculation’’, relying on the fast improvements of compu-ters and software, had foreseen that ‘‘wind tunnel type experiments’’would assuredly disappear before the mythic year 2000. Opticalmethods, considered by the most radical as ancestral means would bewashed away! It is true that a few ‘‘classical’’ methods, a nonpejorative term in this context, do not hide their age. Scientists haverealized since long (see [3] as an example) that the gradient ofrefractive index that accompanies a temperature gradient has theeffect of bending the light. It is this bending which is at the basis ofthe mirage phenomenon. The same bending effect can be producedby a concentration gradient. In fact, William Hyde Wollaston, in theyears around 1800, was among the first to apply diffusion phenom-ena to reproduce a mirage artificially [4]. These refractive indexvariations can be used to characterize a fluid, as Robert Hooke provedas early as 1672 [5,6]. On the other hand, the principle of flow seedingin order to visualize trajectories is attributed to Leonardo da Vinci[7–9]. As a matter of fact, a main international scientific award in thisfield of activities wears his name. Today, the year 2000 and itsaccompanying predictions have been forgotten and the ‘‘numericalwind tunnel’’ has not killed experimental methods.

The scientific evolution has conserved the precious balancebetween theory and experimentation, which is most often a sourceof progress. Even better, the increasing quantity of information

66/$ - see front matter & 2011 Published by Elsevier Ltd.

016/j.optlaseng.2011.10.003

issued from theoretical models and computer processes hasgenerated an important need for validation means, stimulatingthe improvement of existing methods: Shadowgraphy andSchlieren [10], Shearography [11], Interferometry [12] and itsHolographic sister [13] are still topical under entirely renewedaspects. It also induced the appearance of new methods, morerapid, with a best resolution and well adapted to the needs of thespecialists who focus their efforts on either 3D, unstationary,multiphase flows, sometimes all simultaneously.

In another positive way, today’s extraordinary awareness ofenergetic and environmental problems reveals to be a drivingforce behind the development of the characterization of fluidicand thermal phenomena.

Before describing in details the recent developments of FlowVisualization, let us remind that this generic term has knownnumerous evolutions. It first referred mostly to the observation offluid movements, in the way Leonardo da Vinci or his famousheirs such as Reynolds meant it. Let us jump briefly over theintroduction of the quantitative analysis of images since ourglorious ancestors quoted above had already realized it, withneither a laser nor a CCD camera, through positioning, distance ormean velocity measurements and also through the determinationof adimensional numbers.

The measurement of flow parameters is a wide field where wecan distinguish:

–

Purely geometrical parameters: positioning of a vortex orvibration amplitude of a shock wavey[14]

–

Time and speed parameters: velocity fields, but also frequen-cies of periodic phenomenay[15]

–

Thermodynamic parameters: pressure and temperature.

Investigations in a two-phase medium make things morecomplicated because they introduce new parameters: concentra-tion of the two phases, dimensions of droplets, bubbles or othersolid particles carried away by the flows, interface shapey Allthese liable to be time-dependent.

Today, the term ‘‘Flow Visualization’’ synthesizes all these fields,far from a single visual observation. To analyse the recent evolu-tions of this discipline, it is convenient to separate the progressesthat occurred in methods and those realized in applications.

3. Methods

Between the Sixties and the Seventies of the last century, thebirth of lasers as new means of lighting induced a family of newvisualization and measurement methods, as it did in many fields.

Editorial / Optics and Lasers in Engineering 50 (2012) 1–72

For several decades, scientists had mostly used refractiveindex conversions in order to characterize a fluid:

–

Figana

Conversion into a phase signal, itself converted into lightintensity (interferometry).

–

Conversion into a change of direction due to refraction, alsoconverted into light intensity (Shadowgraphy and Schlieren).

These methods, usually called optical methods, very sensitive tothermodynamic parameters (pressure and temperature), hadalready reached a high level of sophistication and gave quantita-tive information. But they did not allow the study of threedimensional flows, except for a few cases showing an axisym-metric character and after a complex reconstruction. Besides, theobservation of tracers carried away by a fluid made up anotherfamily, appreciated for its various results: trajectography bymeans of a dye or of solid particles, velocimetry through thetracking of tracers during a definite time interval, characterizationof wake.

But the low intensity of volumic lightings even in ‘slices’,restricted the level of the scattered light and imposed oversizedtracers, thus of higher inertia. So, the velocity range itself waslimited, often to Hydrodynamics and accelerations difficult toreveal.

The thin sheet of coherent light put an end to this dilemma,allowing far higher energy densities. Moreover, the ‘‘plane byplane’’ analysis opened the way to 3D analysis. Tomography andits quantitative adaptations have at once charmed the specialistsin Fluid Mechanics, those in the industrial world included, because

. 1. Light scattering visualizations : 2D – 3D Evolution. Vortex analysis (a–b) and 3D

lysis, (d) volume analysis. Courtesy Y. Bailly, R. Porcar, J.P. Prenel, FEMTO – ST Inst

the difficult adjustments required by the ‘‘optical methods’’ gaveway to a few easy aligning of lenses (see Fig. 1).

But the very low sensitivity of these tomographic methods vs.thermodynamics parameters made them inefficient in the mea-surements of temperature and pressure. They became the una-voidable basis for the restitution of velocity fields, leaving thesupremacy in the domain of thermodynamics parameters to theuse of refractive index variations (see Fig. 2).

Different types of methods appeared rapidly, allowing:

–

tra

itute

The time tracking of particles, shooting at one time images ofparticles ‘‘frozen’’ in the field, method known as PTV.

–

The analysis of particles tracks whose lengths are variable withthe exposure time or PSV.

–

The correlation between two successive images ‘‘frozen’’ in thefield or PIV [16].

The progresses of image recording systems, of pulsed lightingsystems and of image processing have later allowed a consider-able evolution in speed ranges, field widths and in the number ofextracted vectors.

Lastly, the developing of Laser Induced Fluorescence hasopened a new way in the measurement of thermodynamicparameters, sometimes coupled up with a velocimetry.

Another consequence of the laser introduction was the birth ofSpeckle Metrology [17]. As the operation of the first lasers soonrevealed, a diffuse scattering surface, illuminated by laser light,appears covered by a pattern of bright and dark spots or speckles

randomly distributed in space. Speckles were initially considered as

jectography (c–d) by movable light sheets around a delta wing; (c) sequential

, University of Franche Comte, Parc Technologique - Belfort – France.

Fig. 2. Refractive index visualizations. (a) Polychrome shearing interferometry:

shock wave passing over a diamond cylinder, initial shock Mach number 1.32;

(b) Polychrome Mach-Zehnder interferometry: shock wave passing over a cylin-

der, initial shock Mach number 1.31; (c) Direction-indicating colour schlieren:

shock wave diffracting over vertical edge, initial shock Mach number 1.32.

Courtesy Prof. Harald Kleine, University of New South Wales/Australian Defence

Force Academy, Canberra, Australia.

Editorial / Optics and Lasers in Engineering 50 (2012) 1–7 3

an additional, unwanted, side effect of laser light, and considerableeffort was devoted to find methods for reducing this ’’noise’’. Then,starting from the Burch and Tokarski’s experiment [18], the scientificcommunity turned its attention to the harnessing of the speckleeffect. Today, speckle photography [19,20] is widely used, withrecent applications in tomography [20] and in association withwhite-light sources [21,22]. To a still greater extent, electronic/digitalspeckle pattern interferometry [23] emerged as a very effective andflexible technique in a great variety of engineering applications andalso in flow visualization [24–27].

In quite a different approach, the specialists in Fluid Mechanicshave always taken interest in the near wall phenomena, whichare strongly related to the properties of the fluid and to thecharacteristics of the flow. A revolution happened in this fieldduring the Nineties with the birth of pressure or temperaturesensitive coatings (paints), making possible real cartographies ofsolid–fluid interfaces.

Starting from this summary, which in no way pretends to beexhaustive, we can sketch some tendencies in the flow visualiza-tion research field:

1.

The development of 3D methods goes on through variousapproaches. Tomographic velocimetry – plane by plane ana-lysis – is by nature a 2D analysis, giving access to 2 speedcomponents and called 2D2C. The strong need for 3D data hasinduced the extension of this analysis by using the stereoprinciple which leads to a strong development of the restitu-tion of the 3 speed components near a plane (2D3C analysis), amethod that is well perfected today. The transition to a truemeasurement of the 3 speed components in a volume (3D3Canalysis) is one of the most innovating purposes of these lastyears: colour coding of the third dimension (Rainbow VolumicVelocimetry) [28] or using of the essential property of Holo-graphy (which can also allow to apprehend the dimensions ofthe tracers) are the main ways of approach.

2.

The outstanding come back of Doppler shift had also a greatimpact these last years. The use of the light scattered bytracers in the crossing volume of 2 focussed laser beams,LDV—Laser Doppler Velocimetry (1C, 2C, or even 3C but morecomplex) has reached a high level of performance in term ofspeed range but also of rapidity, giving an easy access to themeasurement of turbulence ratios. Local by nature, it can beused to characterize a volume of fluid only with a sweeping ofthis volume, an unacceptable technique for the analysis ofstrongly variable phenomena. This weakness has stimulatedthe recent emergence of new Doppler methods, operating in afield, the frequency shifts being measured from the imagery ofthis field: DGV—Doppler Global Velocimetry [29], see Fig. 3.As the Doppler shift must be wide enough to be measured ingood conditions, this method is devoted to high speed ranges,for which tracers imagery velocimetries tend to reach theirown limits.

3.

In the field of traditional methods also, those based on thefluid refractive index variations have been subjects to devel-opments and innovations. In particular, the association ofpolychromatic lighting and of holographic interferometry[30] allows to quantify a wide field of refractive indicesthrough the rapid identification of the white fringe whichshows that the optical path is null. The most recent versioncalls for numerical holography [31] and avoids the annoyancesdue to the chemical treatments of the plates.The small deviation of light rays that has been developed forseveral decades in Shadowgraphy and Schlieren [10], oftenwith a qualitative aim, has been ‘reinvented’ and used toextract quantitative information on a refractive index field,sometimes in a 3D area.BOS—Background Oriented Schlieren [32] consists in obser-ving the image of points dispersed randomly behind the indexfield and measuring by correlation the shifts induced bythis field.The current success of this method no doubt comes from thespectacular progresses of automatic spatial correlation sys-tems, developed lastly for PIV. The most recent versions evenintegrate colours, thus becoming the more performing C-BOS.A variant, which can be traced back to Hubert Schardin [33],consists in using a network of parallel lines as a background,

Fig. 3. The laser light sheet visualization of the vertical flow was part of a Doppler

Global Velocimetry (DGV) measurement system. The DGV system is capable of

simultaneously measuring three-components of velocity within the measurement

plane illuminated by the laser light sheet. The investigation of the flow above the

F/A-18 model (1991) was the first use of the technique to measure the flow about

an aircraft configuration. NASA/Courtesy of nasaimages.org.

Editorial / Optics and Lasers in Engineering 50 (2012) 1–74

whose deformations, induced by the index field, can bemeasured by the automatic fringes analysis method, borrowedfrom quantitative interferometry.The near wall investigations quoted above, seemed to belimited, by nature, to simplified analysis, not very quantitativebut cheap and easy to undertake: positioning of threads orobservation of viscous fluids along a wall. It all changed withthe appearing of coatings sensitive to a thermodynamic para-meter. It first began with liquid crystals that could show thelocal temperature as the thin film thermometer on the chil-dren forehead. Then other coatings, in paint form today, haveallowed to reveal local variations of pressure. Often correlated,these two effects made the life of aerodynamics specialistsmore complicated, but the recent improvements in molecules(especially polymers) and in the procedures permit to combinethe two types of response: temperature-compensated pressuremeasurements are now available.This approach has become a technique in itself, even amongmanufacturers (PSP—Pressure Sensitive Paints andTSP—Temperature Sensitive Paints) and reaches today thefield of low differential pressures and partial gas pressures.

4.

The hybridization of different methods is also a new tendency:– Tomographic and near-wall methods join in order to obtain

more complete information inside the fluid and on theinterface;

– The today classical particle image velocimetries are nowcompleted by other information, temperature for example,from the fluorescence induced in tracers. Image velocime-tries can also be coupled with tracers granulometry [34].

– It is now possible to obtain the simultaneous characteriza-tion of a velocity field and of a fuel concentration fieldinside a heat engine model.

This tendency should be intensified in the future, with multi-ple combinations, implementing the various possibilitiesdescribed in this synthesis.

To conclude about methods, the following keywords charac-terize the most recent evolutions: ‘‘True’’ 3D investigationsgiving 3D3C results; Use of colour as a complementary vector of

information; Hybridization of methods; Improvement of thequantitative aspect of the oldest techniques; Improvement oftime resolution, thanks to the technological progresses of pulsedlasers and fast digital cameras.

Lastly, let us keep in mind that Image Processing and Compu-ter Vision are ubiquitous in most of the methods described in thisreport and develop rapidly.

3.1. Applications

A few decades ago, aerodynamics and especially aeronauticsand space industries were the favourite fields of optical flowanalysis. The awareness of the importance of the systematicoptimization of flows and heat transfers rose after the first ‘‘oilcrisis’’. This period coincides with a change in the methodsthemselves.

Optical methods deserved their name since they were mainlyrealized by specialists in optics: the choice of an optical compo-nent, with more criteria than its sole focal distance, the elimina-tion of aberrations, were operations that discouraged non-opticians. The appearance of tomographies, simple to realize,easy to adjust and requiring no quality glass for the windows (apiece of transparent plastic is enough most of the times), changedit all. All the industrial fields that use flows, isothermal or not,were then invaded by qualitative and rapidly quantitative ima-geries: optimization of terrestrial (road or railway) vehicles,internal and external aerodynamics, heating and air-conditioningsystems.

The LDV (Laser Doppler Velocimetry) won its status of indus-trial tool soon afterwards, thus completing the global analysis bylocal measurements with a high temporal resolution. Since then,the success of the optical approach has not lowered, justified bythe global characterization of fields and its non-intrusive aspect.

Other fields of application have appeared progressively: earthand environment sciences, mini and later micro fluidics, lifesciences. A somewhat unexpected application of flow visualiza-tion in the real world is wine optimization, see Fig. 4. For the veryfirst time, classical flow visualization techniques were employedto capture the fluid motion in traditional flutes poured withchampagne in order to underscore the impact of glass shape andglass engravement conditions on the role of flow mixing phe-nomena on the flavours and aroma exhalation process. To betterhighlight the fluid dynamics inside a glass, laser tomographycombined with fluorescent dyes (sulforhodamine B and fluores-ceine) has been used to give the vorticity convection. Once pouredwith champagne, the glass is lighted in its symmetry plane with a1 mm planar laser sheet.

The specific evolution of older – and thus called traditional –domains, has not come to a halt. Under the pressure of energy andenvironment problems, a lot of progresses were made in thetechniques of investigation, allowing their rapid evolution. Manyexamples can be found such as the optimization of combustion,especially in engines and active Fluid Mechanics, essential forflow control.

Another very important field is micro fluidic. Let us recall thatthe usual scales for fluidic and thermal applications are not thoseof opticians for whom micro¼micrometer and nano¼nanometer.A thermal micro-machine is often a system with a global volumeof a few mm3 or even cm3, depending on the field of activity, andthese normalized terms should often be taken in a relative way,according to the interlocutor. So, what does really mean micro-visualization or micro-measurement today?

In the respect of the usual definition of Physics, we can assertwithout hesitation that micrometric dimensions have beenreached: it is possible to observe micrometric gas bubbles andto measure their displacement speed in a boiling liquid as it is

Fig. 4. Flow visualization in a traditional flute poured with Champagne. Because

glasses are usually circularly engraved at their bottom, the resulting flow exhibits

an annular behaviour clearly evidenced. If Champagne remains the wine of

celebration undoubtedly due to its bubbling behaviour, it should also be for the

splendour of its swirling motion. Courtesy Prof. Guillame Polidori, University of

Reims (France).

Table 1Rough guide to flow visualization. Step 1: get acquainted.

References

History [2,5,6,7,10,35,36]a

Collection of images [1,37,38,39]b

Starting resources [40,41,42,43,44]c

Some recent journal special issues [45,46,47,48,49,50,51]

Main conferences [52,53,54,55]

Internet resources [56,57,58]d

a Chapt. 1 of Ref. [10].b Ref. [39] mostly addresses the artistic content of flow visualization

images.c Multi-authored books.d Ref. [58] is no more updated

Table 2Rough guide to flow visualization. Step 2: specialize.

Techniques Main Ref. Reviews Further Ref.

Injection of foreign material [59]a [2,41]

Shadow and Schlieren [59,10] [33,60] [41,61–68]b,c

BOS [32,69] [70] [21,71]d

Interferometry [59,12,73] [41,60,74] [30,75]e,f

Holographic interferometry [12,13,76] [41,74,60] [77,78,79]

Digital holography [31] [80,81]

Shearography [11] [82]

Speckle phot. [17,20] [19] [22]g

ESPI [17,23] [83,84] [25–27,85]h

PIV [17,16,86] [41,87]

Light scattering techniques [41,88,89] [42,90]

Thermography [91] [92]

3D Visualization [93]

3D Velocimetry [94]

a Comprehensive coverage from a single author’s point of view.b Color schlieren, of Ref. [61].c Synthetic schlieren, of Ref. [66].d Compared to colour schlieren, of Ref. [71].e Colour interferometry, of Ref. [30].f Simultaneous visualization by different techniques, of Ref. [75].g White light speckle.h Transient convection from 0.2 to 0.4 m m wires, of Ref. [85].

Editorial / Optics and Lasers in Engineering 50 (2012) 1–7 5

possible to measure the concentrations of two mixing fluids witha micrometric spatial resolution. The ‘‘nanometric’’ scale is notoff-limits anymore: the tracking of particle Brownian movementsis real, at a scale of 10–100 nm.

Furthermore, quite sophisticated methods, often developedin another field of physics, are efficiently adapted to FluidMechanics, such as CLSM (Confocal Laser Scanning Microscopy)and SPR (Surface Plasmon Resonance).

Lastly, Life Sciences today represent a large field of applications,characterized by a strong potential development. All the types ofbiological flows are concerned, including ocular fluid and evencephalo-rachidian liquid movements. For all these, the big challengetoday is to shift from simplified model studies to in-vivo studies.

4. A rough guide to flow visualization

Before briefly introducing the papers of this special issue, wewould like to give a short practical guide to information gatheringin flow visualization. As we showed above, flow visualization is oldand intrinsically cross-disciplinary, therefore related information isspread in a variety of books, journals, films and conferences. The

following rough guide, largely based on personal non exhaustivechoices, is in no way comprehensive but is simply intended to be auseful tool. In particular, Table 1 reports suggested references for afirst approach to the different techniques, while more advanced/specialized references are listed in Table 2.

5. This issue

This special issue, devoted to recent advances in flow visuali-zation, is a collection of 10 papers contributed by some renownedfrontline research groups. It is a calibrated mixture of reports oncurrent state of the art and discussions about new/specializedtechniques with an interdisciplinary approach. We hope this issuewill serve as a useful source of reference, allowing an advance inthe flow visualization field.

The paper by Hargather and Settles gives an excellent per-spective on the modern use of schlieren for quantitative heattransfer measurements. Three schlieren methods are clearlydescribed and applied to the free convection heat transfer alonga vertical isothermal plate.

Desse et al. propose an application of digital colour holographyto assess flow motions (either reacting or not) and mechanicaldisplacements. Using high resolution tri-CCD sensor, the authorsare able to retrieve high spatial and temporal information about

Editorial / Optics and Lasers in Engineering 50 (2012) 1–76

the studied media. The use of colour holography is a good way forsuppressing the ‘‘zero optical path displacement’’ uncertainty, andis therefore a good way to obtain absolute phase maps.

The paper by Sourgen, Leopold and Klatt addresses a problemof current interest: the improvement of the Background OrientedSchlieren technique by the analysis of different colours. This newtechnique is called CBOS (Coloured Background Oriented Schlie-ren), interesting results are presented concerning the 3D mea-surement of the non-axisymmetric density fields around spaceprobe models in supersonic conditions.

Choi, Seo, Sohn and Lee discuss an interesting technique inwhich digital holographic microscopy is coupled with particletracking velocimetry. The authors describe the procedure for thenumerical reconstruction of the optical field from a holographicimage and how the z-position of the seeding particles (in order toreconstruct the 3D field in a microflow) can be detected by usingan autofocus function which quantifies the degree of imagesharpness. After that, the application of PVT algorithms canenable the reconstruction of the particle trajectories in order tohave a complete 3D image of the velocity field. A few applicationsare presented concerning the movement of micro-spheres in amicro-channel, the tracking of red blood cells and the character-ization of the swimming of micro organisms.

The next paper by Coghe and Cossali presents a modern surveyof the experimental diagnostics employed in the investigation ofsprays. Both established techniques (laser scattering, LDV, PIV,PDA) and a number of recently developed techniques (laser sheetdropsizing—LSD, structured laser illumination planar imaging—

SLIPI, molecular tagging velocimetry—MTV, ballistic imaging,X-Ray absorption) are described and discussed.

Huang, Lepiller, Bailly, Guermeur and Herve report on asimultaneous particle size and velocity measurement techniqueusing the Lorenz–Mie principle and the PIV principle, respectively.A brief review of the existing particle sizing methods is given.

Kihm et al. describe the basics of surface Plasmon resonance(SPR) reflectance imaging, giving also details about the setup ofthe SPR reflectance imaging system and the SPR reflectanceimaging resolution. Information about the application of the SPRimaging sensor technique in characterizing the near-field finger-prints of nanofluidic evaporative self-assembly is also given. SPRreflectance imaging could significantly advance flow visualizationwithin 100 nm from the solid surface.

Song, H.D. Kim and K.C. Kim propose a new visualizationtechnique to measure the field of concentration of dissolvedoxygen in a micro scale water flow. The proposed visualizationtechnique is based on the use of a planar molecular transducersensitive to the concentration of dissolved oxygen. Thus, in thepresence of oxygen, it becomes possible to measure the dissolvedoxygen (DO) concentration field in microscale water flows. Theachievable spatial resolution of the acquired concentration mapcould be as high as 3.0 mm.

Kameya et al. report an application of Pressure-Sensitive Paint(PSP) technique with a temperature cancellation method in anon-aerospace problem (pressure measurement on a rotatingHard Disk Drive).

The paper by Young Won Kim and Jung Yul Yoo is devoted tothe reviewing of the advancements in experimental studies onparticle transport in microfluidic channels. These are intended todevelop measurement techniques, flow characterization, and topossibly share future directions.

We would like to use this opportunity to thank the Authors forhaving made this issue possible. We would also like to thank theReferees personally for their efforts and for their valuable comments.

Lastly, along with the information given on the papers pre-sented in this special issue, let us say a few words about thepapers you will not read.

When this special issue was planned, it involved 13 papers:the three ‘‘lost’’ works should have been provided by Japanesecolleagues, which were unable to participate because of theEastern Japan Great Earthquake Disaster that occurred on March,11th 2011.

Warmest wishes to our colleagues, to their families andfriends, as well as to the people of Japan.

References

[1] Van Dyke M. An album of fluid motion. Stanford: Parabolic Press; 1982.[2] Freymuth P. Flow visualization in fluid mechanics. Rev Sci Instrum 1993;64:

1–18.[3] Huygens C. Traite de la lumi�ere, 1690. English translation: Treatise on light.

London: Macmillan and Co.; 1912.[4] The London Encyclopædia, vol. 16. London, Thomas Tegg, 1829, p. 234–235.[5] Rienitz J. Schlieren experiments 300 years ago. Nature 1975;254:293–5.[6] Rienitz J. Optical inhomogeneities: Schlieren and shadowgraph methods in

the seventeenth and eighteenth centuries. Endeavour 1997;21:77–81.[7] Cheng KC. A history of flow visualization: chronology. J Flow Visualization

Image Process 1997;4:9–27.[8] Gharib M, Kremers D, Koochesfahani MM, Kemp M. Leonardo’s vision of flow

visualization. Exp Fluids 2002;33:219–23.[9] Capra F. The science of Leonardo: inside the mind of the great genius of the

Renaissance. New York: Doubleday; 2007.[10] Settles GS. Schlieren and shadowgraph techniques: visualizing phenomena in

transparent media. Berlin: Springer-Verlag; 2001.[11] Steinchen W, Yang L. Digital Shearography. Bellingham: SPIE Press; 2003.[12] Hauf W, Grigull U. Optical methods in heat transfer. Adv Heat Transfer

1970;6:131–366.[13] Vest CM. Holographic Interferometry. New York: Wiley; 1979.[14] Prenel JP, Porcar R, Texier A, Strzelecki A. Dynamic and multiple laser light

sheets : an experimental approach of unstationary and three dimensionalflows, Atlas of Flow Visualization III. CRC Press; 1997. Visualization Society ofJapan, p. 23–41.

[15] Bailly Y, Prenel JP, Guermeur F. Quantitative characterization of acousticphenomena in a resonator by means of particle tracer visualization. In:Mochizuki S. editor, Proceedings of the first pacific symposium on flowvisualization and image processing, Hawaii. Pacific Center for Thermal andFluids Engineering; 1997. p. 645–649.

[16] Raffel M, Willert C, Kompenhans J. Particle Image Velocimetry: a practicalguide. Berlin: Springer-Verlag; 1998.

[17] Sirohi RS, editor. Speckle Metrology. New York: Marcel Dekker Inc.; 1993.[18] Burch JM, Tokarski JMJ. Production of multiple beam fringes from photo-

graphic scatterers. Opt Acta 1968;15:101–11.[19] Kihm KD. Applications of laser speckle photography for thermal flow

problems. Opt Lasers Eng 1998;29:171–200.[20] Fomin NA. Speckle photography for fluid mechanics measurements. Berlin:

Springer-Verlag; 1998.[21] Giglio M, Musazzi S, Perini U. A white light speckle Schlieren technique. Opt

Commun 1981;36:117–20.[22] Ambrosini D, Paoletti D, Di Biase R. White-light speckle photography

technique applied for free convection heat transfer problems. Flow MeasInstr 2010;21:98–104.

[23] Rastogi PK, editor. Digital speckle pattern interferometry and related tech-niques. New York: Wiley; 2001.

[24] Lu B, Yang X, Abendroph H, Eggers H, Ziolkowski E. Measurement of a three-dimensional temperature field applying Espi and CT techniques. Opt Com-mun 1988;69:6–10.

[25] Paoletti D, Schirripa Spagnolo G. Fiber optics speckle interferometer fordiffusivity measurements. J Phys III (France) 1993;3:911–4.

[26] Schirripa Spagnolo G, Ambrosini D, Ponticiello A, Paoletti D. Studying heattransfer with TV holography. Eur J Phys 1994;15:315–8.

[27] Dupont O, Dewandel JL, Legros JC. Use of electronic speckle pattern inter-ferometry for temperature measurements through liquids. Opt Lett1995;20:1824–6.

[28] Prenel JP, Bailly Y. Recent evolutions of imagery in Fluid Mechanics: fromstandard tomographic visualization to 3D Volumic Velocimetry. Opt LasersEng 2006;44:321–34.

[29] Meyers JF, Comine H. Doppler global velocimetry—a new way to look atvelocity. In: Proceedings of the ASME fourth international conference on laseranemometry, Cleveland Ohio; August 1991.

[30] Desse JM. Recent contributions in color interferometry and applications tohigh-speed flows. Opt Lasers Eng 2006;44:304–20.

[31] Schnars U, Jueptner W. Digital holography. Berlin: Springer; 2005.[32] Richard H, Raffel M. Principle and application of the background oriented

schlieren (BOS) method. Meas Sci Technol 2001;12:1576–85.[33] Schardin H. Die schlierenverfahren und ihre anwendungen. Ergebnisse der

Exakten Naturwissenschaften 1942;20:303–439. translated as NASA TTF-12731.

[34] Akiki R, Bailly Y, Lemoine L, Guermer F, Herve P. Simultaneous measurementsof particles sizes and velocities using PIV and scattered polarization ratio.

Editorial / Optics and Lasers in Engineering 50 (2012) 1–7 7

In: Proceedings of the ISFV 13—13th international symposium on flowvisualization, Nice, France; 2008.

[35] Gartenberg E, Roberts AS. Twenty-five years of aerodynamic research withinfrared imaging. J Aircraft 1992;29:161–71.

[36] Durst F. Fluid mechanics developments and advancements in the 20thcentury. In: Proceedings of the 10th international symposium on applicationsof laser techniques to fluid mechanics. Lisbona; 2000.

[37] Nakayama Y, editor. Visualized flow: fluid motion in basic and engineeringsituations revealed by flow visualization. Oxford: Pergamon Press; 1988.

[38] Samimy M, Breuer KS, Leal LG, Steen PH, editors. A gallery of fluid motion.Cambridge: Cambridge University Press; 2004.

[39] Kleine H, Settles GS. The art of shock waves and their flowfields. Shock Waves2008;17:291–307.

[40] Yang WJ, editor. Washington: Hemisphere; 1989.[41] Goldstein RJ, editor. Fluid Mechanics Measurements. 2nd ed.. London: Taylor

& Francis; 1996.[42] Mayinger F, Feldmann O, editors. Optical measurements: techniques and

applications. 2nd ed.. Heidelberg: Springer; 2001.[43] Mercer CR. Optical metrology for fluids, combustion and solids. Dordrecht:

Kluwer Academic Publishers; 2003.[44] Tropea C, Yarin AL, Foss JF, editors. Sprinter handbook of experimental fluid

mechanics. Berlin: Springer; 2007.[45] Ambrosini D, Rastogi PK, editors. Opt Lasers Eng, 44; 2006. p. 155–350.[46] Ambrosini D, Rastogi PK, editors. Opt Lasers Eng, 46; 2008. p. 849–920.[47] Rockwell D, Tropea C, editors. Selected papers from 12th international

symposium on flow visualization. Exp Fluids, vol. 44; 2008. p. 355–490.[48] Romano G, Tropea C, editors. Selected papers from seventh international

symposium on particle image velocimetry. Exp Fluids, vol. 45; 2008. p. 547–780.[49] Clemens NT, Tropea C, editors. Selected papers from 14th international

symposium on applications of laser techniques to fluid mechanics. ExpFluids, vol. 47; 2009. p. 551–881.

[50] Kihm KD, Prenel J-P, editors. Selected papers from 13th internationalsymposium on flow visualization. Exp Fluids, vol. 48; 2010. p. 545–719.

[51] Soria J, Clemens NT, editors. Selected papers from eighth internationalsymposium on particle image velocimetry. Exp Fluids, vol. 50; 2011.p. 775–1182.

[52] International symposium on flow visualization, started Tokyo 1977, accessedon July 4th, 2011, /http://www.isfv.net/S.

[53] International symposium on transport phenomena (ISTP), started Honolulu1985, accessed on July 4th, 2011, /http://www.tuat.ac.jp/�pctfe/S.

[54] Pacific symposium on flow visualization and image processing (PSFVIP),started Honolulu 1997, accessed on July 4th, 2011, /http://www.tuat.ac.jp/�pctfe/S.

[55] International symposia on application of laser techniques to fluid mechanics,accessed on July 12th, 2011, /http://ltces.dem.ist.utl.pt/lxlaserS.

[56] eFluids: a free one-stop resource for fluid dynamics and flow engineering,accessed on July 4th, 2011, /http://www.efluids.com/S.

[57] AerospaceLab, The ONERA electronics journal, accessed on July 4th, 2011,/http://www.aerospacelab-journal.org/S.

[58] Optical methods in heat and mass transfer, accessed on July 12th, 2011,/http://dau.ing.univaq.it/omhat/S.

[59] Merzkirch W. Flow visualization.2nd ed. Orlando: Academic Press; 1987.[60] Kleine H. Measurement techniques and diagnostics, Handbook of Shock

waves, vol. 1. London: Academic Press; 2001.[61] Howes W. Rainbow schlieren and its applications. Appl Opt 1984;23:2449–60.[62] Tanda G. Natural convection heat transfer in vertical channels with and

without transverse square ribs. Int J Heat Mass Transfer 1997;40:2173–85.[63] Tanda G, Devia F. Application of a schlieren technique to heat transfer

measurements in free-convection. Exp Fluids 1998;24:285–90.[64] Jonassen. DR, Settles GS, Tronosky MD. Schlieren ‘‘PIV’’ for turbulent flows.

Opt Laser Eng 2006;44:190–207.[65] Massig JH. Measurement of phase object by simple means. Appl Opt

1999;38:4103–5.[66] Dalziel S, Hughes G, Sutherland B. Whole-field density measurements by

‘‘synthetic schlieren’’. Exp Fluids 2000;28:322–35.[67] Ambrosini D, Paoletti D. Heat transfer measurement by a diffractive optical

element fringe projection. Opt Eng 2007;46. paper # 093606.[68] Ambrosini D, Gorthi SS, Paoletti D, Rastogi PK. Enhancing accuracy and

sensitivity of temperature gradient measurements in DOE Schardin schlieren#2 using windowed Fourier Transform method. In: Proceedings of theinternational conference on advanced phase measurement methods in opticsand imaging. Ascona (Switzerland), AIP Conference Proc. 1236; 2010. p. 124–8.

[69] Venkatakrishnan L, Meier GEA. Density measurements using the backgroundoriented schlieren technique. Exp Fluids 2004;37:237–47.

[70] Kindler K, Goldhahn E, Leopold F, Raffel M. Recent developments in back-ground oriented schlieren methods for rotor blade tip vortex measurement.Exp Fluids 2007;43:233–40.

[71] Elsinga GE, Oudheusden BWV, Scarano F, Watt DW. Assessment andapplication of quantitative schlieren methods: Calibrated color schlierenand background oriented schlieren. Exp Fluids 2004;36:309–25.

[73] Hariharan P. Optical interferometry.2nd ed. San Diego: Academic Press;2003.

[74] Naylor D. Recent developments in the measurement of convective heattransfer rates by laser interferometry. Int J Heat Fluid Flow 2003;24:345–55.

[75] Kleine H, Gronig H, Takayama K. Simultaneous Shadow, Schlieren andinterferometric visualization of compressible flows. Opt Lasers Eng 2006;44:170–89.

[76] Kreis T. Handbook of holographic interferometry. Weinheim: Wiley-VCH;2005.

[77] Jones R, Wykes C. Holographic and Speckle Interferometry. 2nd ed.Cambridge: Cambridge University Press; 1989.

[78] Rastogi PK, editor. Holographic Interferometry. Berlin: Springer-Verlag;1994.

[79] Rastogi PK, editor. Photomechanics. Berlin: Springer-Verlag; 2000.[80] Desse J-M, Picart P, Tankam P. Digital three-color holographic interferometry

for flow analysis. Opt Exp 2008;16:5471–80.[81] Hossain MM, Shaker C. Temperature measurement in laminar free convective

flow using digital holography. Appl Opt 2009;48:1869–77.[82] VanDerWege BA, O’Brien CJ, Hochgreb S. Quantitative shearography in

axisymmetric gas temperature measurements. Opt Lasers Eng 1999;31:21–39.

[83] Doval AF. A systematic approach to TV holography. Meas Sci Technol2000;11:R1–36.

[84] Ambrosini D. Investigation of heat and mass transfer by ESPI. Recent Res DevPhys 2000;1:27–43.

[85] Ambrosini D, Paoletti D, Schirripa Spagnolo G. Study of free-convective onseton a horizontal wire using speckle pattern interferometry. Int J Heat MassTransfer 2003;46:4145–55.

[86] Adrian RJ, Westerweel J. Particle Image Velocimetry. Cambridge: CambridgeUniversity Press; 2011.

[87] Hinsch KD. Holographic particle image velocimetry. Meas Sci Technol2002;13:R61–72.

[88] Prenel JP, Porcar R. Selective polarizing coding laser tomography for aero-dynamics. Opt Comm 1992;89:12–6.

[89] Herve P, Kleitz A, Hamouda A, Leporq B. Determination of submicronicparticle size distribution by means of polarized light: application to con-densation nuclei and turbo reactors. J Aerosol Sci 1994;25:525–6.

[90] Prenel JP. Flow visualization by light scattering: a few historic and scientificaspects, Invited lecture. In: Proceedings of the ISFV 14—14th internationalsymposium on flow visualization. Daegu, Korea; 2010.

[91] Carlomagno GM, de Luca L. Infrared thermography in heat transfer, Hand-book of Flow Visualization. In: Yang WJ, editor. Washington: Hemisphere;1989.

[92] Astarita T, Cardone G, Carlomagno GM. Infrared thermography: an opticalmethod in heat transfer and fluid flow visualization. Opt Lasers Eng 2006;44:261–81.

[93] Prenel JP, Bailly Y. Quantitative imagery for the analysis of complex and 3 Dflows: from tomography to volumic velocimetry. J Flow Visualization ImageProcess 2001;8:189–201.

[94] Zibret D, Bailly Y, Cudel C, Prenel JP, Malfara R. 3D flow investigations bymeans of RVV—Rainbow Volumic Velocimetry: recent progress. J FlowVisualization Image Process 2004;11.

Jean-Pierre PrenelUniversite de Franche Comte, CNRS 6174, Belfort, France

E-mail address: jpprenel@orange.fr

Dario AmbrosiniDIMEG, Universit�a dell’Aquila, Via G. Gronchi 18,

I-67100 L’Aquila, Italy

E-mail address: dario.ambrosini@univaq.itURL: http://laser.ing.univaq.it